1. Field of the Disclosure
The present disclosure, for example, relates to communications systems, and more particularly to measuring voltage standing wave ratio (VSWR) in a radio frequency (RF) communications system.
2. Relevant Background
Microwave or RF transmission systems may have impedance differences between a source or transmission line and a load. Impedance mismatches result in reflection of some of the forward or transmitted power in the opposite direction. The reflected power may result in standing waves along a transmission line. The standing waves may be characterized by a voltage standing wave ratio (VSWR) measurement, which provides an indication of the impedance mismatch of the load.
Accurate measurement of VSWR may be important to various communication systems. For example, certain frequency bands have regulated limits on radiated power, and determining radiated power accurately may depend on accurate measurement of VSWR or return loss. In some cases, it may be difficult to characterize a transmission system accurately and transmitters may be operationally connected to different loads. For example, a Link 16 transmitter may be designed to work in various airplanes and vehicles, and may be connected to different antennas for each application.
Various techniques have been used to measure the VSWR of a system or part of a system. For example, a bidirectional or dual-directional coupler can be inserted in the path which has the VSWR to be measured. The coupler is usually inserted after an amplifier and the path is often common to both transmit and receive, though that is not necessary. The forward power and the reverse power can then both be measured and compared to each other.
However, the directivity of the coupler significantly affects the measurement, as the incident and reflected waves interact with each other causing the voltage vectors of the waves to sum. The amount of error is dependent upon the interacting phase angle, the load impedance, and the power levels of the signals (with the reverse power level being a function of both the forward power, and the load impedance). As a result, the accuracy of using couplers to measure the VSWR of a system typically depends significantly on the directivity of the coupler used for measurement. Therefore, accurate measurement of VSWR provides challenges in RF systems.